1. Field of Invention
This invention relates to sensing the liquid level in a container as used in a beverage brewing device or hot water container in an aircraft.
2. Prior Art
Coffee makers on aircraft are used many times per month and therefore must be very reliable. The current art, U.S. Pat. No. 4,949,627, uses “swingably” mounted electrodes to detect when the coffee maker server is full and to therefore stop filling the server. These electrodes are conductive metal segments linked together by conductive pins, bolts, and nuts. The problem with this design is the fact that over time coffee residue and oxidation builds up between the links and between the pins and links. This residue and oxidation reduces the electrical conductivity of the electrode linkage, and thus renders the electrode sensor inoperable and causing the server to overflow.
One conventional technique for solving the problem of residue interrupting electrical conductivity in electrodes is to use a probe without links. Instead of “swingably” mounted electrodes, probes mounted with springs are used. This provides an electrode without gaps to become contaminated. These probes are always oriented downward into the server. When the server is put into the coffee maker the probes are pushed out of the way by the server and then flex back into position inside the server. The probes are attached to the springs with the axis of the probes aligned with the axis of the spring coils causing the springs to flex perpendicular to the axis of the spring.
Repeated bending and overstressing of the springs causes failure of the springs and the probes to sever and detach. The current art, U.S. Pat. No. 7,017,408, attempted to eliminate the mechanical failures associated with spring mounted probes by replacing conductive probes with infrared light emitting diodes (IRED) to detect the liquid level in the server. The IRED concept has problems with the lenses becoming dirty from steam and coffee residue which cause the IRED concept to not detect the liquid surface and therefore overfilling occurs. The IRED also has difficulty detecting the liquid surface when the surface is rippled due to aircraft vibration. The IRED concept also has errors detecting the maximum allowed surface level when the aircraft is not in level flight since the liquid surface is not horizontal. Problems also occur with the IRED concept in detecting the liquid surface due to the density of steam. All of these problems cause the IRED concept to overflow the server.